This invention relates to a laser-transmissible composition including a dynamic crosslinking polyolefin thermoplastic elastomer for forming of a laser-transmissible molded workpiece and a method for laser welding of the molded workpiece made from it.
A thermoplastic elastomer is a synthetic polymerized material having a property of plastic like a thermoplastic resin that softens under high temperature, and a property of rubber that exhibits elasticity under room temperature. The thermoplastic elastomer can perform injection-molding as same as the thermoplastic resin to form moldings having various shape. Further it attains excellent productivity and recycling. Therefore it is used generally as materials for the moldings that takes midway roles between the rubber and the plastic such as the thermoplastic resin in extensive fields of automobile industry, electric industry, electronic industry and so on.
The thermoplastic elastomer includes mostly the rubber component as a soft segment such as styrene-butadiene copolymer rubber (SBR), ethylene-propylene-diene-methylene copolymer (EPDM) and acrylonitrile-butadiene copolymer rubber (NBR). Furthermore it includes the thermoplastics component as a hard segment such as polyolefins, polystyrenes, poly(vinyl chlorides), polyesters, polyamides and polyurethanes.
Especially, a polyolefin thermoplastic elastomer (TPO) is most used as the material for moldings because it is available at low manufacturing cost and has excellent properties such as tensile strength, elongation at break and tension set. The polyolefin thermoplastic elastomer includes the plastic component such as the thermoplastic resin of polypropylene and polyethylene, and the rubber component such as polyolefin. It is prepared by merely blending, polymerization, or crosslinking.
For joining mutually molded workpieces made from the thermoplastic resin such as polypropylene, a method for laser welding is known. The laser welding is executed for example as described in Japanese Patent Provisional Publication No. 11-170371. A laser-transmissible molded workpiece made from the thermoplastic resin and a laser-absorptive molded workpiece made from a thermoplastic resin are put together. Laser is irradiated from the laser-transmissible molded workpiece side to transmit. The laser transmits to the laser-absorptive molded workpiece, is absorbed thereto and causes the exothermic. Both molded workpieces are melted at put portion thereof together by the exothermic to weld. For executing the laser welding with sufficient welding strength, it is necessary that the laser-transmissible molded workpiece has high laser-transmittance to cause sufficiently transmitting of the laser and then the exothermic.
However the polyolefin thermoplastic elastomer tends to have lower laser-transmittance than the thermoplastic resin such as polypropylene and polyamide having a sufficient laser-transmissible property, because it has been denaturalized by blending, polymerization or closslinking.
Further the polyolefin thermoplastic elastomer is gradually oxidized or deteriorated to yellow. It is necessary to be colored beforehand with pigment of colorant, for making the yellow inconspicuous or for classifying orderly by coloring.
A composition including the pigment must have the still lower laser-transmittance because of the pigment that immensely obstructs the laser-transmissible property. As consequence, there are a few problems. When the laser-transmissible molded workpiece made from the polyolefin thermoplastic elastomer is executed with the laser welding, it is welded without the sufficient strength, in comparison with using the laser-transmissible molded workpiece made from the thermoplastic resin.
Further if the laser welding thereof is executed by the considerable strong output of the laser to make up for the lower laser-transmittance and to transmit the sufficient laser, overheating on the surface of the molded workpiece causes burnout, deterioration and spoiling of smoothness.
On the other hand, the laser-transmissible molded workpieces made from the polyolefin thermoplastic elastomer colored with known oily dyestuffs that do not obstruct the laser-transmissible property so much instead of the pigment of the colorant, are easy to be discolored under molding and welding. Further if the welded workpieces are left under high temperature and high humidity, the dyestuffs cause remarkably a bleeding property (i.e. a migrating property) from the laser-transmissible molded workpiece into the laser-absorptive molded workpiece made from the thermoplastic resin or the polyolefin thermoplastic elastomer. As consequence, it causes spoiling of appearance.